Silicon aluminum oxynitride refractory/ceramic materials, and more particularly materials in the Si.sub.3 N.sub.4 --AlN--Al.sub.2 O.sub.3 --SiO.sub.2 system, are of ever-increasing interest for refractory/ceramic applications. For ease of identification, compositions within this system are referred to as SiAlON, and a number of different phases of SiAlON have been produced and identified. For example, Jack et al U.S. Pat. No. 3,991,166 describes one phase and methods of making it, the phase having the general formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z is greater than zero and less than or equal to five. Various compositions within the bounds of the general formula taught by Jack et al may be produced, and each has a crystalline structure similar to beta-Si.sub.3 N.sub.4 and is consequently identified as beta'-SiAlON (.beta.'-SiALON). .beta.'-SiAlON can be defined as a solid solution of Al.sub.2 O.sub.3 within a matrix of Si.sub.3 N.sub.4. The compositional limits of reactants, referred to as effective reactants, to produce .beta.'-SiAlON may be seen by referring to FIG. 2. The compositional amounts of Si.sub.3 N.sub.4, AlN and Al.sub.2 O.sub.3 for any .beta.'-SiAlON formulation may be determined by referring to line AB which is a plot of the compositions of the aforesaid compounds to produce a .beta.'-SiAlON having the general formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z is greater than zero and less than or equal to five.
Another phase, known as y-phase SiAlON represented by the formula SiAl.sub.4 O.sub.2 N.sub.4, is described in an article entitled "Review: SiAlONs and Related Nitrogen Ceramics", published in Journal of Material Sciences, 11 (1976) at pages 1135-1158. Compositions of SiAlON within a given phase and from phase to phase demonstrate varying characteristics, for example, variances in density, which effect their preferential use in a given application.
A number of processes for making silicon aluminum oxynitride refractories and technical ceramics have been suggested. Weaver U.S. Pat. No. 3,837,871 describes a method for producing a product having a substantial amount of what the patentee believes to be the quaternary compound silicon aluminum oxynitride which has a structure similar to that of beta Si.sub.3 N.sub.4 but with an expanded lattice structure. Weaver's method of making the described product is hot pressing Si.sub.2 ON.sub.2 (silicon oxynitride) in the presence of varying amounts of aluminum.
Kamigaito et al U.S. Pat. No 3,903,230 describes a method of making a silicon aluminum oxynitride ceramic by sintering or hot pressing a mixture of finely divided powders of silicon nitride, alumina and aluminum nitride.
Cutler U.S. Pat. No. 3,960,581 describes a process for producing SiAlON by reacting silicon and aluminum compounds in the presence of carbon and nitrogen. Cutler teaches and stresses the importance of using a reactant material having the silicon and aluminum compounds intimately combined prior to nitriding in order that aluminum oxide is intimately dispersed throughout silicon nitride in the final product. Suggested reactant materials are clay, rice hulls having a solution containing a dissolved aluminum salt absorbed therein, and a precipitate of aluminum and silicon salts. In each case Cutler emphasizes that the silicon and aluminum compound reactants are intimately combined prior to nitriding to produce SiAlON. Further, in the process as taught by Cutler, excess carbon and unreacted silicon dioxide must be removed from the mixture after the mixture is nitrided.
Maeda U.S. Pat. No. 4,172,108 describes a process for production of SiALONs which involves heating a mixture containing a silicon nitride precursor having at least one silicon-nitrogen bond and an alumina precursor having at least one aluminum-oxygen bond to at least 1000.degree. C.
Inoue U.S. Pat. No. 4,680,278 describes a process for preparing aluminum nitride powder having small particle size and small particle size distribution and also having a uniform shape of particles, at a lower temperature and in a shorter period of time. Inoue teaches that the aluminum nitride powder can be mixed in predetermined amounts with silicon carbide and silicon nitride to form SiAlON.
Thus far, of all the SiAlON materials, the .beta.'-SiAlONs have generated the greatest interest because their refractory properties and corrosion resistance characteristics are comparable to other nitride refractories such as silicon nitride and silicon oxynitride. More recently, the .beta.'-SiAlONs have generated a great deal of interest as technical ceramics, i.e. monolithic engineered ceramics.
.beta.'-SiAlON compositions offer a distinct advantage over silicon nitride and silicon oxynitride for making a refractory because some of the compositions of .beta.'-SiAlON material can be used to produce a high density refractory by conventional sintering techniques. To produce high density ceramics from silicon nitride or silicon oxynitride requires the use of pressure sintering techniques.
Jack et al U.S. Pat. No. 3,991,166 describes a .beta.'-SiAlON product produced by sintering a mixture of alumina or a compound which decomposes to produce alumina and silicon nitride. Another method of producing .beta.'-SiAlON as described by Jack et al is nitriding silicon powder in the presence of alumina powder.
Demit U.S. Pat. No. 4,147,759 describes a method of manufacturing .beta.'-SiALON compounds. The method involves reacting silicon nitride and aluminum oxynitride in the presence of an agent which generates gaseous silicon monoxide.
It may be noted that several of the foregoing processes employ silicon nitride or silicon oxynitride as reactants. Neither of these compounds is found in nature and they are relatively expensive to produce. The production of SiAlON from discrete particles of an SiO.sub.2 source and discrete particles of an Al.sub.2 O.sub.3 source requires a catalyst to enhance both reaction rates and stoichiometry so as to make the process inexpensive and economically attractive.
Phelps et al U.S. Pat. No. 4,499,193 describes a process for carbothermically producing an unsintered refractory material comprising essentially .beta.'-SiAlON wherein the initial reactants include discrete particles of an SiO.sub.2 source and discrete particles of an Al2O3 source. Phelps discloses that it is advantageous to add iron in the form such as Fe.sub.2 O.sub.3 as a catalyst in promoting the formation of .beta.'-SiAlON. The .beta.'-SiAlON material produced according to the teachings of this patent have the microstructure of .beta.'-Si.sub.3 N.sub.4 and sinter into a material that has an equiaxed microstructure.
Phelps et al U.S. Pat. No. 4,511,666 describes a process for carbothermically producing an unsintered refractory material comprising essentially .beta.'-SiAlON wherein initial reactants include discrete particles of an SiO.sub.2 source, discrete particles of an Al.sub.2 O.sub.3 source and discrete particles of silico alumina compounds. The initial reactants are nitrided for sufficient times and temperatures to convert at least a portion of the initial reactants to at least a portion of effective reactants, and the effective reactants are then further heated to produce an essentially .beta.'-SiAlON refractory material. Phelps discloses that it is advantageous to add iron in the form such as Fe.sub.2 O.sub.3 as a catalyst in increasing the rate of reaction and promoting the formation of .beta.'-SiAlON. Phelps also discloses that oxides of other transitional metals such as nickel, chrome or manganese, for example, may also be used as catalysts. The .beta.'-SiAlON material produced according to the teachings of this patent have the microstructure of .beta.-Si.sub.3 N.sub.4 and sinter into a material that has an equiaxed microstructure.
Generally, only a small percentage of catalyst, such as 2% or less Fe.sub.2 O.sub.3, for example, is added to increase the rate of carbothermic reaction and reduce the length of time needed to form SiAlON. However, when Fe.sub.2 O.sub.3 is used as a catalyst, the iron reacts with silica to produce an FeSi phase which is present as a contaminant in the final sintered product. FeSi forms flaw sites in the SiAlON which initiate fractures and lowers its room temperature strength. Furthermore, when the SiAlON is used in high temperature (1200.degree.-1300.degree. C.) applications, the FeSi oxidizes and further reduces the materials strength.
The use of .beta.'-SiAlONs as refractory/ceramic materials is the result of their ability to maintain superior strength, hardness, creep resistance and resistance to chemical attack at elevated temperatures (above 1000.degree. C.). Co-pending U.S. Ser. No. 351,660 discloses a process for producing an unsintered SiAlON material by carbothermic reaction without the use of contaminating transition metal oxides to increase the rate of reaction. The process includes providing small quantities of SiAlON crystals which seed the reaction. The absence of FeSi in the .alpha.'SiAlON produced a material with an increased the high temperature strength.
Increasing demands in the refractory/ceramic industry as creating a need for materials with increased strength at high temperatures. It would be advantageous, therefore, to provide a process whereby readily available and relatively inexpensive initial reactant materials comprising Al.sub.2 O.sub.3 and SiO.sub.2 are nitrided to make silicon aluminum oxynitride materials having a strength greater than 50 ksi at 1000.degree. C.
Another object of the present invention is to provide a low-cost process for producing SiAlON from initial reactant materials comprising Al.sub.2 O.sub.3, SiO.sub.2 and carbon that does not require the addition of transition metals such as iron, nickel, chrome or manganese, to be used as catalysts to increase the rate of reaction.
A further object of the present invention is to provide a process for producing unsintered .beta.'-SiALON powder by carbothermic reaction.
These and other objects and advantages will be more fully understood and appreciated with reference to the following description.